Enhancing Power Transmission Stability with HVDC Systems During Load Contingencies Akram N. Merzah 1,2* , Abbas H. Abbas 1 , Firas M. Tuaimah 2 1 Electrical Engineering Department, University of Basrah, Basraha 00964, Iraq 2 Electrical Engineering Department, University of Baghdad, Baghdada 00964, Iraq Corresponding Author Email: akram.alali@coeng.uobaghdad.edu.iq Copyright: ©2024 The authors. This article is published by IIETA and is licensed under the CC BY 4.0 license (http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.18280/jesa.570118 ABSTRACT Received: 12 October 2023 Revised: 19 January 2024 Accepted: 30 January 2024 Available online: 29 February 2024 The transmission network of a power system is important in connecting interactions between the generation and distribution sides. A significant aspect in the power system profile is voltage improvement. This study intends to examine the impact of inserting High Voltage Direct Current (HVDC) on the system's voltage stability, network power losses and power transfer capacity of transmission network under several cases of load contingency. IEEE 57-Bus test system is used for testing the addition of HVDC transmission based on genetic algorithm. Modeling of point-to-point HVDC transmission and multi-terminal HVDC transmission is carried out using the Power System Simulator for Engineering (PSS/E) version 32 Package Program (A collection of computer programs and organized data files called PSS/E software was developed by Siemens PTI to handle the fundamental tasks of power system performance simulation work). The system's performance was compared with and without the HVDC inserted under different loading scenarios: 5%, 10%, and 20% of the total load. The comparative results can show that active power losses at the normal load case are reduced by 55.714% after inserting point to point HVDC topology, and after inserting multi-terminal HVDC topology reduced by 68.214%. Also, the reactive power losses reduce by 55.714% after inserting point to point HVDC topology and after inserting multi-terminal HVDC topology reduced by 66.830% at the same case. The results shown that inserting HVDC Transmission to the system gives better improvement in bus voltage profile and a significant reduction in total network power losses and increase in power transfer capacity of transmission network. The results also showed that multi-terminal HVDC transmission is better in voltage improvement and total power losses reduction when HVDC Transmission is added to the system. Keywords: voltage improvement, HVDC, network power losses, load contingency, PSS/E 1. INTRODUCTION In order to transfer significant amounts of electrical energy, high voltage alternating current (HVAC) systems have been in use for more than a century [1]. However, typical HVAC transmission systems have certain limitations, including lowered reliability in transporting electrical energy, issues with environmental, efficiency, losses in power, cost of construction, and the system's voltage stability, which is of the highest priority [2-4]. Since a significant portion of failures in a power system are caused by voltage instability, voltage stability has been recognized as an essential need for a power system to operate safely and reliably [5]. Due to the weakness of the HVAC system, massive power losses observed in long- distance transmission lines [6] are what lead to the majority of voltage stability issues. Therefore, the usage of High Voltage Direct Current (HVDC) to enhance AC system networks' quality and reliability has been based on the requirement to enhance transmission capacity as well as reduced plant running costs. This is because the HVDC system has the advantage of overcoming the limitations of the AC systems [7]. The reactive power consumption issue that occurs during line charging, especially on cable lines HVAC, is resolved by HVDC. Reactive power in transmission lines, which limits the power transmission capacity in HVAC technology and increases the current which led to increase transmission losses, is not a problem in the HVDC system. Comparing this to AC high-voltage lines, more power can be transmitted over longer distances at a lower cost. HVDC systems allow for the connection of different electrical systems and regions divided by a large water area, as well as the connection of continental regions with island regions (such as the UK with Europe). Also, they are used in linking power systems in situations where frequency maintenance is problematic or there are different frequencies [8]. Even while renewable energy sources have many advantages, their intermittent nature and the distance between major installations and demand centers have a significant impact on the stability of the connected grid. In the presence of large renewable energy installations, HVDC systems are used as a solution for reliable and stable grid operation, this is a significant benefit of HVDC, when offshore areas employ this technology [9]. Hybrid HVAC/HVDC transmission systems provide several advantages over HVAC transmission systems, such as a better voltage profile, lower cost of losses, system stability, better efficiency and management, and lower short circuit current levels [10, 11]. Journal Européen des Systèmes Automatisés Vol. 57, No. 1, February, 2024, pp. 177-185 Journal homepage: http://iieta.org/journals/jesa 177